So, how do you know if Safari is still running in the background? Just look at your Dock (see Dock screenshot below and note the red arrows). Safari is still running in the background and will prevent Adobe Flash from being installed. Instead, they have closed the active window and they don’t see Safari on the screen anymore and assume, incorrectly, that Safari is no longer running at all. The vast majority of my clients experience this error because they have not properly quit Safari before trying to install Adobe Flash.
They tell me that the installation only goes a little more than halfway before failing. Many clients ask me why it is that they cannot install Adobe Flash on their Macs.